Multiple burners are widely employed in industrial boilers, such as those used in conjunction with steam turbines for electric power generation. These burners may be fired by a variety of fuels such as coal, oil or gas and usually have an associated supporting igniter for initial combustion of the fuel. It is necessary to monitor the flame on these burners to ensure that flame is present at all times during the operation of the burner. In the event of a flame failure, a burner may continue to supply fuel resulting in a potentially hazardous situation. Occasionally, a burner may not ignite upon start up. Therefore, it is required that such conditions be immediately identified and prompt remedial action taken.
Over the years, a variety of flame detection devices for monitoring burner fires and for providing an output based on the presence or absence of flame have been developed and employed. A well known detection method is to use an optical device to examine the light emitted from the flame. A typical optical flame device consists of a light sensitive sensor that generates a time varying voltage when exposed to light. In most prior art flame detection devices, the sensor is a single discrete element, allowing only the overall light intensity to be represented in the spatial region of interest.
Several techniques have been developed to examine sensor output and control the burner system. Such conventional systems directly process the magnitude of time varying output voltage of the sensor, which is directly proportional to the light intensity. As the light intensity increases, so does the magnitude of the output voltage. This level is analyzed to determine the presence or absence of flame on the burner of interest.
Devices employing this technique and variations thereof have several disadvantages. For instance, in multiple burner systems, a flame sensor is placed on each burner and tuned to detect the flame of that particular burner only. Often the background flame from adjacent burners will have the same or greater intensity as that of the burner of interest. This background intensity may cause the output of the optical sensor to remain at a level expected in the case when flame is present, even though the burner may be shut down. The detector will then incorrectly indicate the presence of flame. This is a common problem, since conventional detectors have difficulty with flame discrimination under these circumstances.
Improvements in flame detection results have been obtained by post processing the time varying output into the frequency domain and then analyzing the frequency spectral characteristics of the flicker rather than the limited time domain voltage, as disclosed by Davall et al. in U.S. Pat. Nos. 4,983,853 and 5,107,128. However, the sensor used in this method is a single discrete element, and only allows for the overall light intensity to be detected in a defined spatial region.
Additionally, such conventional devices do not have the ability to sense multiple fuels due to spectral wavelength limitations of the individual sensors. If the fuel type is changed, the sensor must be switched to detect the different ultraviolet, visible or infrared spectra associated with the new fuel.
There is accordingly a need for an improved system that overcomes the limitations associated with using a single elemental optical flame detector, particularly the deficiencies found in their flame discrimination capability, and thereby increase the user's confidence level in the detection of flame in industrial scale fuel burner applications.